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A dumb question

lostone

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Will a spinning celestial object, such as Earth, ever stop rotating? After all, doesn't spinning do work, and take energy out of the system? In which case, doesn't the spin of electrons and protons also do work?

The same goes for orbits, some of which decay. Although in the case of the moon, it is gradually moving away from Earth.
 
Will a spinning celestial object, such as Earth, ever stop rotating? After all, doesn't spinning do work, and take energy out of the system? In which case, doesn't the spin of electrons and protons also do work?

The spinning itself doesn't do work. Angular momentum is a conserved quantity unless acted upon by an outside torque. Given that, there are torques acting on Earth, hence why the spin does change as well as the orientation of the rotational axis (precession). These are very, very small relative to the total angular momentum of the Earth, so if the Earth were to ever completely stop spinning it likely will be on universal timescales.


The same goes for orbits, some of which decay. Although in the case of the moon, it is gradually moving away from Earth.
As the Moon moves away from the Earth, the angular momentum of the system is pretty much conserved and there's an according slowing down of the Earth's rotation. It is said that once the Moon gets far enough out and the Earth slows down enough so that the month and the day are the same length, then the transfer of angular momentum through tidal torques will stop.
 
Then, in a sense, is spinning of celestial objects, such as neutron stars, perpetual motion, since only outside forces can stop it or slow it down?
 
The idea of "perpetual motion" as an exceptional and unattainable condition is medieval, and relies on the notion that motion requires an energy input to be sustained.

That's a local truth. If you live at the bottom of a gravity well, with an atmosphere, and you are so completely under the influence of those things at all times that you don't even notice them, then such "laws of nature" as 'perpetual motion without an external force is impossible', or 'nature abhors a vacuum', are easy to demonstrate.

When you realise that the vast majority of everything IS vacuum, your perspective needs to change, or your ideas will become obsolete.

A spinning object in isolation will spin forever.

A planet is not entirely isolated, of course. Tidal effects from the Sun, Moon, and other planets and even distant stars and galaxies, all act to modify the rotation of the Earth. But the distances are large, gravity is weak, and the influence is inversely proportional to the square of the distance, so it will take a LONG time for any effects to become significant.

The tides slow the rotation of the Earth. Tne Moon is the largest tidal influence, because although it's small, it's very, very close by (in cosmic terms), at only half a million miles. The Sun is also a noticable influence; it's far away, but it's very large. No other object has an effect we can easily measure, or can detect without highly specialist equipment, over short timescales (and by 'short' I mean 'less than the time that human record keeping has existed').

So yes, in a sense, the spinning of an isolated neutron star is "perpetual motion", in that the motion will continue indefinitely.

But in the sense that the complete phrase "perpetual motion" implies an unknown and supernatural influence to counter the supposed natural tendency for all motion to stop, which is how the phrase is most commonly used in philosophy, it is an erroneous concept, based on the medieval mistake of ascribing local, (earthbound) conditions and observations to the universe as a whole.

Medieval observers saw the Moon and the Sun revolving around the world, and couldn't understand why they didn't slow down like everything else. Worse still, the planets occasionally did slow down, but then speeded up again! Trying to explain this led to all kinds of weird and arbitrary hypotheses (google "epicycles" if you want to know more about the knots these guys tied themselves in, trying to explain what they could see).

Three simple (but not at all obvious) ideas did away with the philosophical need for supernatural "perpetual motion"; The hypothesis that the universe is mostly vacuum; The hypothesis that the Earth is itself in motion, and not the fixed centre of the universe; And the hypothesis that an object in motion will continue in motion unless acted upon by a force.

As all moving objects near the Earth's surface are acted upon by forces applied by the air or water through which they move, and as we cannot feel the motion of the Earth, these three ideas were very hard to accept, and very difficult to demonstrate. But we now know them all to be the case. And, impressively, the likes of Copernicus, Galileo, Newton, and Kepler were able to demonstrate them before we had even achieved flight, much less spaceflight. Now we can actually go and see for ourselves that space is mostly vacuum - but we knew it before we went, just because of a handful of really impressive thinkers who were able to interpret the very mixed signals we were getting from our observations, which were the result of our abnormal situation: On a planet with an atmosphere, in orbit around a star, and orbited by a large and nearby Moon.
 
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A spinning object in isolation will spin forever.
B-but there is no isolation afaik. Everywhere in the universe the object will be bombarded by photons and worse, and be subject to gravitational flux and fields, no matter how faint.
 
A spinning object in isolation will spin forever.
B-but there is no isolation afaik. Everywhere in the universe the object will be bombarded by photons and worse, and be subject to gravitational flux and fields, no matter how faint.
Yes. And?

A spinning object a few light weeks from a star, or a few light hours from a planet, will spin for longer than anyone will be around to care about it. It will still be spinning when the stars all go out. Even if it explodes as a supernova, or collapses into a neutron star, its angular momentum (including that of all the fragments) will remain virtually unchanged indefinitely.

Sure, it will pick up odd random variations from impacts with distant-source cosmic radiation; But these will mostly tend to even out over time - half will speed it up, and the other half will slow it right back down. Any net effect will be tiny. It will experience tidal deceleration from the nearest masses, but they are a long way off, so the amount will be utterly negligible. Not zero, for sure; But close enough for government work.
 
A spinning object a few light weeks from a star, or a few light hours from a planet, will spin for longer than anyone will be around to care about it. It will still be spinning when the stars all go out.
Forever? Upon the supposed heat death of the universe, will things still be spinning relative to each other?
close enough for government work.
I can be confident of that. 😆
 
Forever? Upon the supposed heat death of the universe, will things still be spinning relative to each other?
All I know is that a spherical cow, at absolute zero, in a perfect vacuum, at an infinite distance from any other mass, will spin forever.
How could you tell?

Anyhow, first you need a spherical cow.
 
A spinning object a few light weeks from a star, or a few light hours from a planet, will spin for longer than anyone will be around to care about it. It will still be spinning when the stars all go out.
Forever? Upon the supposed heat death of the universe, will things still be spinning relative to each other?
The thing to ask isn't whether it'll stop spinning, but rather, what force is acting on it to stop spinning? The second Newtonian Law states that stuff in motion will stay in motion until something fucks its motion up. The Third Newtonian Law stats that no... that was the first Newtonian Law. So if something is in motion, it needs something to slow it down.

A lot of people don't get that the issues with motion and work on the surface of Earth have many more barriers than in space. Things work against action on the surface, wind resistance, friction, illegal immigration, transgender issues, and electrostatic force among other things. In a vacuum, these factors aren't applicable.
 
Jimmy nailed it.


In physics and engineering, a free body diagram (FBD; also called a force diagram)[1] is a graphical illustration used to visualize the applied forces, moments, and resulting reactions on a free body in a given condition. It depicts a body or connected bodies with all the applied forces and moments, and reactions, which act on the body(ies). The body may consist of multiple internal members (such as a truss), or be a compact body (such as a beam). A series of free bodies and other diagrams may be necessary to solve complex problems. Sometimes in order to calculate the resultant force graphically the applied forces are arranged as the edges of a polygon of forces[2] or force polygon (see § Polygon of forces).



A ball falling to Earth has downward force exerted by gravity and aerodynamic drag opposing the force.

Sketch a falling ball. Draw an arrow pointing up from the top to represent drag and an arrow from the bottom pointing down down representing gravity.

One picture is worth a thousand words(or a thousand posts).

Heat indicates work being done. If heat is being creted then energy is being consed and somewhere there is an energy debit.

Same question I posed about a Foucault Pendulum. The pendulum swings and there is r friction in the mechanism to overcome. Does the heat generated in friction show up as a debit in Earth's angular momentum?
 
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The pendulum swings and there is r friction in the mechanism to overcome. Does the heat generated in friction show up as a debit in Earth's angular momentum?
Perplexity sez

The heat generated by the pendulum's friction does not directly affect Earth's angular momentum. The reasons for this are:
  1. Scale: The energy involved in the pendulum's motion is infinitesimally small compared to the Earth's rotational energy.
  2. Closed system: The pendulum and the Earth form a closed system. The energy conversions happening within this system (from mechanical to thermal) do not change the system's total angular momentum.
  3. No external torque: For the Earth's angular momentum to change, an external torque would need to be applied. The internal friction of the pendulum does not provide such an external torque
The explanation is longwinded but that looks like the nut of it.
 
Then, in a sense, is spinning of celestial objects, such as neutron stars, perpetual motion, since only outside forces can stop it or slow it down?

On stuff like this I resort to Laws Of Thermodynamics.

Draw a thermodynamic boundary around the star. Define mass and energy in and out of the boudary. If there are net losses then states inside the boundary have to change.

A homogeneous metal ball with no external forces is evensong in space.If the spin is not perfect and there is wobble here will be frictional forces inside the ball as it tres to deform. Energy is lost through termal rediation, does the ball slow down?
 
The thing to ask isn't whether it'll stop spinning, but rather, what force is acting on it to stop spinning?
PERPLEXITY DAY...

Conservation of Angular Momentum​

Despite the energy loss through thermal radiation, the ball's angular momentum must be conserved in the absence of external torques. This leads to an intriguing behavior:
  1. The ball does not slow down in the conventional sense.
  2. Instead, it tends to align its rotation with the axis of maximum moment of inertia

The Dzhanibekov Effect​

This behavior is related to the Dzhanibekov Effect, which describes the strange instabilities of rotating bodies

For a homogeneous ball, all axes have the same moment of inertia, but any slight imperfection or initial wobble can trigger this effect.
(bold added)
 

A homogeneous metal ball with no external forces is evensong in space.If the spin is not perfect and there is wobble here will be frictional forces inside the ball as it tres to deform. Energy is lost through termal rediation, does the ball slow down?
what do you mean by “perfect” and “wobble”? why would the spin not be “perfect” if there are no external forces or torques on the ball?

If by “wobble” you don’t mean precession but something like acoustic oscillations would those not be damped by the internal forces to which you refer and eventually stop, thus eventually leaving the ball spinning oscillation free at a fixed angular velocity?
 
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Will a spinning celestial object, such as Earth, ever stop rotating? After all, doesn't spinning do work, and take energy out of the system? In which case, doesn't the spin of electrons and protons also do work?

The same goes for orbits, some of which decay. Although in the case of the moon, it is gradually moving away from Earth.
Our physics intuitions tend to be Aristotelian: that every objects tries to find its "natural" place and motion.

But there are various things that can affect the Earth's rotation:
  • Tidal drag - by far, the most significant
  • Drag by the solar wind on the Earth's magnetosphere
  • Electromagnetic waves from the Earth's magnetic field's rotation
  • Gravitational waves from mountains and the like on the Earth's surface
 Future of Earth - The Earth's rotation is being tidally dragged by the Sun and the Moon, and the Moon is being pulled outward by the Earth pulling forward its tides.

If the Earth survives the Sun's red gianthood, then in several billion years, the Earth will be dragged enough to enter synchronous rotation with the Moon, with one side always facing the Moon, like one side of the Moon always facing the Earth. But the Earth will still be subject to tidal drag from the Sun, and that will slow down its rotation further. That will make the Moon's tides on the Earth pull the Moon inward, and the Moon will be pulled inward until after several billion more years, it gets very close, it breaks apart, and it becomes a ring around the Earth.
 
The second Newtonian Law states that stuff in motion will stay in motion until something fucks its motion up. The Third Newtonian Law stats that no... that was the first Newtonian Law.
I think you'll find that Newton's first law states:

"An object in motion shall not harm a human being, nor through inaction allow a human being to come to harm"
 
Things work against action on the surface, wind resistance, friction, illegal immigration, transgender issues, and electrostatic force among other things. In a vacuum, these factors aren't applicable.
Though, to be fair, a lot of modern vacuums have a HEPA filter as well as a bag, so that might change things. And then there's the Dyson design that doesn't have a bag at all - it's like some kind of weird vacuum with nothing in it at all, if you can imagine that.
 
I take the descent into abject punnery as strong evidence that the question has been adequately answered.
 
The pendulum swings and there is r friction in the mechanism to overcome. Does the heat generated in friction show up as a debit in Earth's angular momentum?
Perplexity sez

The heat generated by the pendulum's friction does not directly affect Earth's angular momentum. The reasons for this are:
  1. Scale: The energy involved in the pendulum's motion is infinitesimally small compared to the Earth's rotational energy.
  2. Closed system: The pendulum and the Earth form a closed system. The energy conversions happening within this system (from mechanical to thermal) do not change the system's total angular momentum.
  3. No external torque: For the Earth's angular momentum to change, an external torque would need to be applied. The internal friction of the pendulum does not provide such an external torque
The explanation is longwinded but that looks like the nut of it.
Yes the effect may be immeasurable by our instruments, but unless you abandon LOT and conservation there is an effect.

LOT and conservation applies to a finite volume within the universe not necessarily the universe in toto whatever it may be. And that leads into cosmology and origins, again.
 
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